Prior Work – CompAir 8 Leading Edge Skins

This was one of my first Rhino projects done in 3D from start to finish, and the first project that utilized CNC water jetted plywood forms with steel studs to hold everything in alignment. The leading edge of this CompAir 8 was horribly misshapen, and it’s hard to figure out how anyone could not have that problem. Per the factory, the wings are tapered, and the ribs are hand cut. Instead of using a molded and cored skin, they expect you to wrap some pre-made fiberglass sheets (just tri-ax fiberglass cloth and vinyl ester resin I believe) around these hand shaped ribs, and somehow end up with a nice leading edge shape. Well, not surprisingly, when the customer tried to do that, that did not happen at all. We looked at various methods of fixing the existing skins – creating bodywork templates and such, but in the end concluded that it would be far better to create new leading edge skins from scratch, and to make them molded. The idea was thrown around to make a mold from CNC water jetted plywood, and the customer, coming from a drywall and steel stud background, suggested we use four steel studs to keep everything aligned. A few days spent in front of the computer, and I had the CAD work done. The mold would look something like this:

Since the wing was straight lofted, we figured we could use sheet metal as the skin for the mold. Since aluminum was deemed too expensive, we had a local metal fab shop gently tack weld some sheets together, which turned out to be a mistake, although a fairly minor one in the grand scheme of things. The labor to have the sheets welded together and delivered was equal to what we would have spent just getting some nice aluminum on a roll, and it took the better part of two weeks for them to deliver the metal.

While waiting for the sheet metal to show up, I took 5 or 6 sheets of 3/4″ finish grade plywood down to our local metal fab shop RCR Fabrication. A half day later we had 28 perfectly cut formers. Someone suggested that we grab the “drop” from the cuts, which is the part that gets cut out and typically left behind. That turned out to be a fantastic idea. The plywood forms and steel studs were assembled in about a day. As you can see, two of the studs are “horizontal” and two are “vertical,” which meant we had strength in both directions. Since the slots in the formers were CNC cut, they fit like a dream and the mold practically self aligned. Another neat feature of this technique is that the mold does not need to be put on a flat reference table. All that is needed to make sure there is no twist in the mold – a quick check with a level at a few stations along the mold is all that’s needed. The mold was shimmed into alignment and then Bondo’d onto the floor. At this point the mold was looking like this:

When the sheet metal finally showed up, that decision to keep the drop really paid off. We used the drop and a ton of clamps to gently and slowly drive the sheet metal into the mold. Once the metal was fully seated in the mold, it was glued to the formers. Bondo was used on one, but we found that caulk style construction adhesive worked far better, and was faster to apply since it didn’t need to be catalyzed.

After that it was just a matter of prepping the mold with wax and release agent, and laying up the parts. The fabric used was a tri-ax carbon that weighed in at something like 16 oz./square yard. The core was 1/4″ urethane foam, and the resin was a vinyl ester, which was selected for fuel proofing reasons. While it was pointed out by more than a few that there are plenty of fuel proof epoxies and that the fabric selected was needlessly heavy, those of that opinion were overruled. This represented the first, and last time I’ll ever work with vinyl ester resin. I have done plenty of work with polyester resins, but I have to say this stuff is even worse when it comes to fumes. Even with masks, we all ended up with resin hang overs the days after doing big layups. All in all though, the part came out fine, albeit heavier than it needed to be. Each skin was around 17 feet long, and if I remember right they each weighed around 45 pounds.